The United States has concluded that Russia violated a landmark arms control treaty by testing a prohibited ground-launched cruise missile, according to senior American officials, a finding that was conveyed by President Obama to President Vladimir V. Putin of Russia in a letter on Monday.

It is the most serious allegation of an arms control treaty violation that the Obama administration has leveled against Russia and adds another dispute to a relationship already burdened by tensions over the Kremlin’s support for separatists in Ukraine and its decision to grant asylum to Edward J. Snowden, the former National Security Agency contractor.